There has been considerable interest in recent years in the development of a monitoring system for obtaining a continuous measurement of a patient's blood pressure. One of the most promising techniques for obtaining such a continuous measurement involves the use of an arterial tonometer comprising an array of small pressure sensing elements fabricated in a silicon "chip." The use of such an array of sensor elements for blood pressure measurements is disclosed generally in the following U.S. Pat. Nos. 3,123,068 to R. P. Bigliano, 3,219,035 to G. L. Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick, 4,269,193 to Eckerle, and 4,423,738 to P. M. Newgard, and in an article by G. L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med. Elec., Apr. 1963, pp. 73-81).
In a typical tonometric technique for monitoring blood pressure, a transducer which includes an array of pressure sensitive elements is positioned over a superficial artery, and a hold-down force is applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured, and the transducer is positioned such that more than one of the individual pressure-sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. One method for selecting the pressure sensitive element for monitoring blood pressure is disclosed in the above mentioned U.S. Pat. No. 4,269,193 issued to J. S. Eckerle. In addition, an improved method for selecting the correct pressure sensitive element for measuring blood pressure is disclosed in a patent application entitled "Active Element Selection for Continuous Blood Pressure Monitor Transducer" filed on even date herewith.
One of the difficulties encountered in prior systems for employing tonometric techniques for blood pressure measurement is the provision of an appropriate pressure source for creating the hold down pressure necessary to bring the sensor into contact with the patient's arm and to optimally flatten the underlying artery. In particular, prior designs based on conventional pump assemblies create pressure transients which tend to interfere with the operation of the pressure sensing elements. In addition, conventional pressure sources tend to be difficult to control, thus making it difficult to maintain a constant hold down pressure for the tonometer transducer. The pressurization system of the present invention, described in greater detail below overcomes these difficulties.